1. Field of the Invention
The present invention relates to a magnetic recording medium for use in, for example, a magnetic disk system. More particularly, the present invention pertains to a magnetic recording medium which employs a ferromagnetic metallic film as a recording layer and which has excellent corrosion resistance, a small value of modulation that represents the degree of low frequency variation of read output, a lowered level of noise generated during read and write operations, improved durability, excellent magnetic properties and high reliability and is therefore suitable for high density recording. The invention is also concerned with a process for producing the abovedescribed magnetic recording medium.
2. Description of the Prior Art
Conventional magnetic recording media which employ as recording layers ferromagnetic metallic films formed by means of evaporation, sputtering, ion plating or various other deposition methods have excellent magnetic properties suitable for high density recording but, at the same time, suffer from the following disadvantage. Since the magnetic layer is constituted by a metallic film, it is readily corroded and the magnetic properties are thus degraded with time, which means that this type of magnetic recording medium has inferior durability and reliability. In order to overcome this disadvantage, a multilayered magnetic recording medium has been proposed in Japanese Patent Publication No. 33523/1979. In this magnetic recording medium, a thin film which is formed from a metal such as Cr, Ti, Mn or V or an oxide thereof is provided as an underlayer (intermediate layer) or an overcoat for a ferromagnetic metallic film in order to improve the corrosion resistance. As another solution, a method wherein 2 to 15% by weight of Cr which is a third element is added to a magnetic thin film of Co or Co--Ni alloy in order to improve the magnetic properties and durability is described in Japanese Patent Laid-Open No. 15406/1982. In these conventional magnetic recording media, corrosion resistance is substantially improved, but the improvement in magnetic properties is still insufficient. Accordingly, they are required for magnetic recording media for high density inferior in terms of performance and reliability which are recording. Recently, a magnetic recording medium which has less lowering in read and write efficiency, reduced friction and no fear of the surface being shaved and which is excellent in both traveling performance and durability has been proposed (see Japanese Patent Laid-Open No. 223025/1985) in which a topcoat (lubricant layer) which contains a lubricant, an oxidation inhibitor, an organic binder, etc. is formed on a ferromagnetic metallic film by means of coating, and the surface roughness (Rmax) of the topcoat is adjusted so as to fall within the range from 20 to 200 .ANG.. However, this prior art mainly relates to magnetic recording tapes, and the topcoat layer serving as a lubricant layer is provided mainly for the purpose of reducing the friction occurring at the surface of the magnetic tape to thereby improve the traveling stability.
As described above, efforts have been made to improve the durability and magnetic properties of conventional magnetic recording media which employ ferromagnetic metallic films as magnetic recording layers: namely, provision of an underlayer (intermediate layer) or an overcoat (protective layer) on a magnetic film; addition of a third element such as Cr to a magnetic film; and formation of a lubricant layer having a specific surface roughness for the purpose of minimizing friction and thereby improving the traveling stability. However, many of these proposed magnetic recording media are intended to be used mainly in the audio, video or digital recording field. Therefore, although the corrosion resistance and durability are substantially improved, the magnetic properties are still insufficient to satisfy stringent specifications for magnetic recording media used for high density recording in which higher magnetic properties, durability and reliability are required, for example, hard disks for computers. Read and write characteristics of magnetic recording media include so-called modulation (modulation of read-signal envelope) which is a value represented by a parameter [(Emax-Emin)/(Emax+Emin)] which is determined by a maximum output value Emax and a minimum output value Emin of a read-signal envelope curve on the same circumference in read and write operations of a magnetic disk. It is known that the value of modulation has a considerable effect on read and write errors, and it is therefore strongly desired to lower the value of modulation.
As one example of ferromagnetic metallic films, a thin film which contains Co as a principal component and 20 to 30 at (atom) % of nickel (Ni) is employed. This thin film is formed by, for example, the sputtering method, and a magnetic recording medium having a basic cross-sectional structure such as that shown in FIG. 10 is produced. In an ordinary process, an Ni-P amorphous layer 102' is formed on an Al (aluminum) alloy substrate 101' by means of electroless plating, and a Cr layer 103', a recording layer 104' and a protective layer 105' are sucessively formed thereon to construct a magnetic recording medium. Since the recording layer 104' which contains Co as a principal component has a hexagonal crystallographic structure, the crystalline orientation of the recording layer is dependent on the sputtering conditions. For example, if such a recording layer is formed on a glass substrate, the C-axis of crystal tends to orient perpendicular to the film. In the longitudinal magnetic recording system in which recording is effected by magnetization in a direction parallel to the direction of travel, such a tendency is undesirable since the C-axis [0001] which is the magnetic easy axis orients perpendicularly to the substrate 101'.
The Cr layer 103' under the recording layer 104' is an underlayer which is formed in order to cause the C-axis of the recording layer 104' to extend horizontally. The effect of the Cr layer 10' is discussed in 1-200 of "Collection of Lecture Papers at General National Meeting of Electronic Communication Society (1985)". The Cr layer 103' which is formed by the sputtering method orients perpendicular pendicularly to the (110) plane, and when the Co--Ni recording layer 104' is successively formed thereon, the recording layer 104' is epitaxially grown on the Cr layer 103', and a (1100) plane appears parallel to the film surface. More specifically, the C-axis of the recording layer 104' which is the magnetic easy axis is formed so as to extend horizontally. As a result, the in-plane coercivity Hc becomes high, i.e., about 700 Oe, but the magnetic recording medium in which the c-axis of the recording layer 104' which is the magnetic easy axis extends horizontally has the problem that the noise generated in read and write operations is increased.
As described above, in the prior art no consideration is given to the favorite crystallographic orientation of the recording layer, and the prior art therefore suffers from unfavorably large noise in read and write operations.